1. Technical Field of the Invention
The present invention relates to a laser irradiation apparatus and a method of irradiating laser light for crystallizing, recrystallizing, or activating after ion implantation the semiconductor film and the like with laser light. In addition, the present invention relates to the laser irradiation apparatus and the method of irradiating laser light for irradiating a poly-crystalline or near poly-crystalline semiconductor film with the laser light so as to enhance its crystallinity. In addition, the present invention relates to a method of manufacturing a semiconductor device including the laser irradiation process. It is noted that the semiconductor device described here includes a general device that can be operated by utilizing the semiconductor characteristic. And thereby an electro-optical device such as a liquid crystal display device and a light-emitting device, and an electronic device having such an electro-optical device as its component are also included in the semiconductor device.
2. Description of the Related Art
In late years, a technique to form TFT over a substrate makes great progress, and application development to an active matrix semiconductor display device is advanced. In particular, TFT with the poly-crystalline semiconductor film is superior in field-effect mobility (what is called mobility) to TFT with a conventional amorphous semiconductor film and thereby high-speed operation becomes possible. Therefore, the technological development has been promoted for controlling a pixel by a driver circuit formed on the same substrate as the pixel, which was conventionally controlled by the driver circuit provided outside the substrate.
By the way, as for the substrate to be used in the semiconductor device, a glass substrate is expected rather than a single-crystal silicon substrate in terms of cost. The glass substrate is inferior in heat resistance, and is easy to be transformed due to the heat. Therefore, when a poly-silicon TFT is formed over the glass substrate, laser annealing is often employed for crystallizing the semiconductor film in order to prevent deformation of the glass substrate due to the heat.
The characteristics of laser annealing are that the processing time can be drastically shortened compared with annealing method using radiation heating or conduction heating, that the semiconductor substrate or the semiconductor film can be heated selectively and locally so that thermal damages are hardly given to the substrate and so on.
It is noted that the laser annealing method here indicates a technique to recrystallize an amorphous layer formed over the semiconductor substrate or the semiconductor film, and a technique to crystallize the amorphous semiconductor film formed over the substrate. In addition, a technique applied to planarize or modify the surface of the semiconductor substrate or the semiconductor film is also included.
In addition, in the laser annealing process, a pulsed laser beam having a high output such as excimer laser is shaped into a square spot having a side of several cm, or into linear having a length of 10 cm or more through an optical system, and then the irradiation position of the shaped beam spot is relatively scanned to the surface to be irradiated. Since such a method can enhance productivity and is superior industrially, it is employed preferably. (For example, Japanese published unexamined application H08-195357 is referred to).
In particular, when the linear beam spot is employed, unlike a punctate beam spot requiring to be scanned from front to back and from side to side, the linear beam spot can provide high productivity since the linear beam spot can be irradiated to the large-sized surface to be irradiated by scanning the linear beam spot only in a direction perpendicular to the direction of the major axis of the linear beam spot. It is noted that the linear beam spot here means a rectangular beam spot having a high aspect ratio. The reason why the beam is scanned in the direction perpendicular to the direction of the major axis of the linear beam spot is that it is the most effective scanning direction. Because of such high productivity, at present, the laser annealing process is mainly employing the linear beam spot obtained by shaping the beam spot output from a pulsed excimer laser through an appropriate optical system.